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70                                              IODINE

gas is passed through the flask and the hydrosulphite solution run in, gradu-
ally and with shaking, until the blue colour of the indigo is replaced by
a greenish-yellow colour. Dividing 1000 by the number of c.c. of hydro-
sulphite solution used, the percentage of Na2S,,O4 in the siibstance tested
is obtained.

(6) WITH SODIUM SULPHOXYLATE (hydrosulphite-formaldehyde). The
indigo solution is prepared as before, but with 1-705 grams of indigo. To
100 c.c. of this solution are added 10 c.c. of glacial acetic acid and the liquid
heated for 5 minutes, illuminating gas being passed and the solution titrated
with a solution of the sulphoxylate (10 grams per litre) as before. The
percentage of NaliSOo, CH,() + 2HaO is obtained by dividing 1000 by
the number of c.c. used.

With zinc sulphoxylate., 14 grams are dissolved in cold water together
with 90 grams of ammonium chloride and Go c.c. of 25% ammonia, the
liquid being filtered and made up to a litre. This solution is used for the
titration of 100 c.c. of the indigo solution (1-705 gram per litre), to which
are added 10 c.c. of acetic acicl, us above. If ;/ is the number of c.c. of the
sulphoxylate solution used, the percentage of /inc. snlphoxylutc (ZnHS02,
CH20)20 is given by 10930/14 X n.

IODINE

I -----126-92 (127)

Crude iodine forms small crystals or brown crystalline musses, while
the resublimctl product is in dark grey rhoinboidul plates with metallic
lustre ; when heated it yields violet vapour. It dissolves in about 10
parts of alcohol and is soluble in carbon clisulphide giving a violet solution
and also in potassium iodide solution. The most, common impurities art;
moisture, chlorine, bromine, cyanogen, small proportions ol' (ixed sub-
stances, and graphite. The tests to be made are :

1.  Moisture.—When shaken in a dry glass vessel, moist iodine becomes
attached here and there to the walls.

To estimate the moisture, about 0-5 grain of the iodine is placed in a tube
i cm. wide and 6 cm. long, 2-3 grains of powdered silver being then added
and the whole weighed and gently heated until all the water is expelled
(the upper layer of .silver should remain unchanged), cooled and reweighed.

2.  Fixed Substances .—-i gram is heated slowly in a pom-lain dish
until all the iodine is volatilised, the residue being weighed and examined
(mineral substances, graphite).

3.  Cyanogen, Chlorine,   Bromine.—About   i   gram   of   the   well
powdered iodine is triturated with about 40 c.c. of water, the liquid being
decanted off and divided into two portions :

(a] To one portion is added sufficient dilute sodium thiosnlphuto to
decolorise it, then a crystal of ferrous sulphate, two drops of ferric chloride
and a little soda ; after heating, the liquid is acidified with hydrochloric
acid. In presence of cyanogen, a coloration or precipitate of prussian blue
appears.ng